This is a study of te phenomenon of adaptation in invertebrate neurons. Adaptation refers to the decrement in firing rate of a neuron subjected to a constant depolarization. We have found in Aplysia neurons in which adaptation occurs in fast and slow phases. The fast adaptation results from the decay of an inward current over the first 10 seconds of firing. Slow adaptation develops over any tens of seconds and results from a slowly increasing potassium conductance. Both fast and slow adaptation may depend on the level of free intracellular calcium.